How can we decide if any particular measurement near zero is
actually zero or represents a true positive count? There must be some level above which we can be confident, to a degree,
that a net count is valid. This is the critical limit, L_{C}.
If we have a count, A, then when
A >
k_{a }s_{0},
the count is statistically significant
and when
A £
k_{a }s_{0},
the count is not significant, where s_{0}
is the standard deviation of the distribution and the factor k_{a}
is selected so as to provide a predetermined degree of confidence
in the solution.
When k_{a} corresponds to the
95% confidence level
where s is the standard deviance of
background estimate (counts).

Definition of critical limit, L_{C}.
(The vertical axis represents the frequency of observing a particular
count) 